Spoil backward animal (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'backward animal' is the wordplay.
'backward' is a reversal indicator.
'animal' becomes 'ram' (male sheep).
'ram' reversed gives 'MAR'.
(Other definitions for mar that I've seen before include "Disfigure" , "hurt" , "Cause harm" , "Blight" , "Ram into and spoil" .)